Approximate Analysis of Sub-synchronous Oscillation for Grid-connected VSC System Induced by Unilateral Limit

To the sub-synchronous oscillation(SSO) caused by the power electronic equipment, the current researches mainly focus on the small-signal model, which does not account for limit, or the approximate model considering symmetry limits, but they rarely consider the impact of asymmetric limit. In this paper, for the sub-synchronous oscillation phenomenon of the voltage source converter (VSC) system induced by the asymmetric unilateral d-axis current limit (AU-d-CL, or the upper saturation limit), the describing function of the unilateral limit and the generalized Nyquist criterion are used for the approximate analysis. Specifically speaking, firstly, a simplified model of the grid-connected VSC system is established. The small-signal stability property of the grid connected VSC system is analyzed, and the phenomenon of the negative damping SSO induced by AU-d-CL is presented. Furthermore, the frequency-domain model of th eVSC system and the describing function of the AU-d-CL are deduced. Finally, combined with the describing function and the generalized Nyquist criterion, the amplitude and frequency of the SSO with different parameters and limit values are approximately estimated, the results of which are basically consistent with the simulation results. It may explain the phenomenon that the amplitude of the SSO varies when the limit values vary while the frequency remains unchanged. © 2022, Power System Technology Press. All right reserved.